1. Field of the Invention
The present invention relates to a system and method for processing materials, and more particularly to a system and method for densification of a thermal spray coating deposited on a substrate material.
2. Discussion of Related Art
Friction stir welding (FSW) is a welding technique invented by The Welding Institute (TWI) in 1991. FSW is a solid-state joining process that is a combination of extruding and forging. FSW works at a temperature below the melting point of the work piece material.
Referring to FIGS. 1A and 1B, FSW uses a cylindrical, shouldered tool 101 that is rotated and plunged into a joint line 102 between two pieces of sheet or plate material 103–104, which are butted together. The tool 101 comprises a nib 105 and is constructed from a suitable material, e.g., having suitable thermochemical stability, hot hardness, and abrasion resistance. Frictional heat is generated between the tool 101 and the pieces of work material 103–104. This heat causes the work piece material to soften and allows the tool 101 to traverse the joint line 102, wherein the plasticized work material is transferred from a leading edge 106 of the tool 101 to the trailing edge 107 of the tool shoulder and nib. The tool 101 creates a solid phase bond 108 between the two pieces of work material 103–104.
During welding, three flow fields may exist: a primary rotational flow field due to the rotating sliver of material surrounding the nib; a secondary translational flow field created as a sliver of metal entrained in the threaded nib traverses the substrate; and a tertiary vortex field forming smoke rings adjacent to the sliver of entrained metal as a result of the primary and secondary flow fields. This model helps to explain the intercalated, lamellar, or onion-like structures of a friction stir weld. The normal force (Forceplunge=6ΠRτ(T); where R is the radius of the shoulder and τ is the temperature dependent shear flow stress) and the moment (Power=2ΠR2tτ(T)ω; where R is the shoulder radius, t is the plate thickness, τ is the temperature dependent shear flow stress, and ω is the angular velocity of the weld tool=2Π(rpm)/60) needed for welding and translational forces (ForceTranslational=12Rtτ(T), where R is the shoulder radius, t is the plate thickness, and τ is the temperature dependent shear flow stress) produced during welding can be estimated.
Since its invention, FSW has emerged as an important solid state joining process for applications in industries including aerospace, shipping, railway, as well as military.
Thermal spray is a directed spray process in which material in molten form, semi-molten, or solid form, is accelerated to high velocities, and impinged upon a substrate, where a dense and strongly adhered deposit is built. Material may be injected in the form of a powder, wire or rod into a high velocity combustion or thermal plasma flame, which imparts thermal and momentum transfer to the particles. By carefully controlling the plume characteristics and material state, it is possible to deposit a vast range of materials (e.g., metals, ceramics, polymers and combinations thereof) onto virtually any substrate in various conformal shapes. For metals, the particles can be deposited in solid or semi-solid state. For ceramic deposits, the particles may need to be well above the melting point, which is achieved by either a combustion flame or a thermal plasma arc. The deposit is built-up by successive impingement of droplets, which yield flattened, solidified platelets, referred to as ‘splats’. The deposited microstructure and, thus, properties, aside from being dependent on the spray material, depend on the processing parameters, which can be numerous and complex.
However, no know system or method exists for processing a thermal spray coating using friction stir welding. Therefore, a need exists for a system and method for processing a thermal spray coating and a substrate by friction stir welding.